The invention relates in general to security systems which allow operation upon the receipt of a properly coded signal. More particularly, the invention relates to a security system or to a barrier operator system, such as a garage door operator, employing a transmitter and a receiver which communicate via code streams having at least a portion thereof which changes with multiple operation of the device.
It is well known in the art to provide garage door operators or other barrier operators which include an electric motor connectable through a transmission to a door or other movable barrier which is to be opened and closed. Since many of these systems are associated with residences, as well as with garages, it is important that opening of the barrier be permitted only by one who is authorized to obtain entry to the area which the barrier protects. Some garage door operator systems have in the past employed mechanical lock and key arrangements associated with electrical switches mounted on the outside of the garage. While these systems enjoy a relatively high level of security, they are very inconvenient to use for a person because it necessitates them exiting their vehicle in order to send the command to open the garage door. This also may present some danger to people when the exit the relative security of their vehicle if someone may be waiting to do injury to them.
It is also well known to provide radio-controlled garage door operators which include a garage door operator unit having a radio receiver and a motor connected to be driven from the radio receiver. The radio receiver is adapted to receive radio frequency signals or other electromagnetic signals having particular signal characteristics which, when received, cause the door to be opened. More recently, such transmitter and receiver systems have become relatively more sophisticated in that they use radio transmitters which employ coded transmissions of multiple or three-valued digits, also known as "trinary bits" or other serial coded transmission techniques. Among these systems are U.S. Pat. No. 3,906,348 to Willmott, which employs a transmitter and receiver system wherein a plurality of mechanical switches may be used to set a stored authorization code.
U.S. Pat. No. 4,529,980 to Liotine et al. discloses a transmitter and receiver combination for use in a device such as a garage door operator wherein the transmitter stores an authorization code which is to be transmitted to and received by the receiver via a radio frequency link. In order to alter or update the authorization code contained within the transmitter, the receiver is equipped with a programming signal transmitter or light emitting diode which can send a digitized optical signal back to the transmitter where it is stored. Other systems also employing encoded transmissions are U.S. Pat. Nos. 4,037,201, 4,535,333, 4,638,433, 4,750,118 and 4,988,992.
While each of these devices have provided good security for the user, it is apparent that persons wishing to commit property or person-related crimes have become more sophisticated as well. It is known in the security industry today that devices are being made available that can intercept or steal rolling code.
Transequatorial Technology, Inc. sells integrated circuit code hopping encoders identified as Keeloq Model NTQ105, NTQ115, NTQ125D and NTQ129. Some of the keeloq code hopping encoders generate serial codes having fixed portions, i.e., which do not change with repeated actuation of the encoding portion of the chip and rolling code portions which alter with each actuation of the encoding portion of the chip. In order to avoid, however, having the problem of the encoding portion of the chip having been inadvertently enabled and causing the rolling code to be altered on successive enabling attempts thereby leading to a rolling code which is transmitted and not recognized by a receiver, the keeloq code hopping encoders provide a window forward system, that is they are operable with systems having code receivers which recognize as a valid code not a single rolling code, but a plurality of rolling codes within a certain code window or window of values which are the values which would be generated on a relatively small number of switch closures as compared to the total number of rolling codes available. The problem with such a system, however, might arise if a user was away for a period of time or had inadvertently caused codes to be transmitted excluding the number of codes normally allowed within the valid forward code window. In that case, the rolling code would not be recognized by the receiver and the user could not gain entry without taking other measures to defeat the locking system or the garage door operator system which might involve the intervention of a trained engineer or technician.
Texas Instruments also has a prior system identified as the Mark Star TRC1300 and TRC1315 remote control transmitter/receiver combination. The system involves the use of a rolling code encoder which increments or rolls potentially the entire code, that is it does not leave a fixed portion. The system also includes a forward windowing function which allows an authorized user to be able to cause the receiver to be enabled within a limited number of key pushes. Like the keeloq system, if the forward window is exceeded, the Texas Instruments system must be placed in a learn mode to cause the system to relearn the code. In order to place the system into the learn mode, the person must obtain direct access to the receiver to cause a programming control system associated with the receiver to be hand actuated causing the receiver to enter a learn mode. Once the receiver has learned the new code, the receiver will then construct a new valid forward code window within which valid rolling codes may be received. The problem, of course, with such a system is that if, for instance in a garage door operator, the only portal of entry to the garage door is through the overhead door controlled by the garage door operator, the user will not be able to obtain entry to the garage without possibly having to do some damage to the structure. This problem is sometimes referred to in the industry as a "vaulted garage."
What is needed is an economical encoding system which provides good security by using a rolling code, but which enables a user of the system to proceed via a gradually degraded pathway in the event that the receiver detects a signal condition indicative of what might be a lack of security.